The present invention generally relates to staged seal assemblies, particularly for extreme duty applications in pumps, mixers, etc. More particularly, the present invention relates to a staged seal assembly having multiple staged seals and an external pressure breakdown device connected to an external device. Even more particularly, the seal assembly may be configured to be installed in a new installation or replace an existing seal assembly (e.g., a non-staged seal assembly) without requiring modifications to existing, external systems originally connected to the replaced seal assembly.
Staged seal assemblies are known, wherein the seal assembly includes multiple staged seals in series along a rotating shaft to reduce an operational pressure to a fraction of its original value in order to reduce the severity of the seal duty, to provide operational redundancy (i.e., seal redundancy), or to meet a certain seal leakage criteria. The reduction or division of pressure is established by a flow path with a controlled pressure reducing mechanism or device (e.g., orifices, length of coiled tubing, valves, etc.), which is placed in parallel with each seal stage. The flow through the parallel path is typically much greater than the seal leakage such that normal variations to the seal leakage of any stage results in only small changes in the breakdown of the staging pressure(s) (sometimes referred to as staging “stiffness”). However, conventional staged seal assemblies do not contain an external pressure breakdown device, let alone, one configured to connect to external devices or systems. External pressure breakdown device, as defined herein, is a pressure breakdown device additional to the one pressure breakdown device per staged seal found in staged seals.
Moreover, current staged seal assemblies are not configured to replace existing non-staged seal assemblies without requiring modifications to external devices or systems. Non-staged seal assemblies are mechanical seals that include multiple seals in series along a rotating shaft but lack the parallel flow path that allows a portion of fluid to flow through a pressure breakdown device. Therefore, non-staged seal assemblies do not have controlled pressure breakdown across each seal. The pressure breakdown device is variable, depending upon the ratio of one seal leakage to another and the external boundary conditions. Generally, the non-staged seals have very high seal leakage (e.g., 1-8 gal/min in extreme duty applications) due to the severe pressure across the seal (e.g., 2250 psi at the first seal). Such non-staged seal assemblies have increased seal wear and maintenance requirements and lack redundancy found with staged seals.
Non-staged seals were placed into service in extreme duty applications. Due to the high leakage and increased risk of seal failure due to the pressure at the seal, particularly at the first seal, external devices were connected to the seal assemblies such as monitoring/measuring equipment, volume control tanks, etc. These systems were designed for the specific fluid flow parameters (e.g., pressure and flow) of the flow at the location where the external device is connected to the seal assembly. For example, many non-staged seal assemblies had external devices such as a flow monitor or volume control tank connected to them after the first seal such that the leakage past this first seal could be monitored or controlled before moving onto the second seal. Thus, the external device was designed for that specific fluid's flow parameters.
This is problematic when replacing or retrofitting a non-staged seal. It is desirable to replace the non-staged seal assembly with a staged seal assembly to achieve the increase reliability and redundancy of staged seals over non-staged seals. However, a staged seal has different fluid flow parameters after each of its staged seal versus the random fluid flow parameters found after each of the non-staged seals. Therefore, if one would replace a non-stage seal assembly with a staged seal assembly, it would generally require replacing or modifying all the existing external equipment originally connected to the non-staged seal assembly (e.g., fluid flow monitoring equipment, volume control tanks, etc.) so that the external device's fluid flow capabilities match the fluid flow of the new staged seal assembly. These modifications, particularly within primary coolant pumps of nuclear facilities, can be extensive and expensive to undertake. This extensive and costly undertaking has prevented companies from replacing existing, non-staged seal assemblies with staged seal assemblies, thus preventing the companies from achieving the benefits of staged seals (e.g., increased seal life and redundancy). Moreover, conventional staged seal assemblies are not designed to have external channels that feed external devices connected to the system diverting some of the fluid from the staged seal assembly to the external device while maintaining approximately equal pressure breakdowns across each staged seal of the seal assembly.
Accordingly, there is a need for improved staged seal assemblies.